Bimodal polyethylene for blow-moulding applications

Abstract

The invention relates to A process for the polymerisation of ethylene to produce a polyethylene resin in at least two slurry loop reactors connected to each other in series, the resin having a bimodal molecular weight distribution, a molecular weight distribution MWD of at least 7.0, an HLMI of from 1 to 100 g / 10 min, and a density of from 0.935 to 0.960 g / cm3,wherein in one reactor 30 to 47 wt % based on the total weight of the polyethylene resin of a high molecular weight (HMW) polyethylene fraction is produced having an HL275 of from 0.05 to 1.8 g / 10 min (the equivalent of HLMI of from 0.01 to 1.56 g / 10 min), a density of from 0.925 to 0.942 g / cm3 and an MWD of at least 5.0,and in the other reactor a low molecular weight (LMW) polyethylene fraction is produced having an HLMI of from 10 to 1500 g / 10 min and a density of from 0.960 to 0.975 g / cm3,in the presence of a Ziegler-Natta catalyst system.

Description

FIELD OF THE INVENTION[0001]This invention relates to a process for polymerizing ethylene in the presence of a Ziegler-Natta catalyst system in two reactors in series to produce a polyethylene resin having a bimodal molecular weight distribution with a high extrudate swell and impact properties for blow-moulding applications. In particular, the invention relates to polyethylene resins suitable for making blow-moulded articles, such as open-head and tight-head drums, fuel tanks, and jerrycans.BACKGROUND OF THE INVENTION[0002]Polyethylene has desirable properties that have helped to make it the highest volume polymer manufactured. Ziegler-Natta catalysts are a mainstay for polyolefin manufacture and are used in slurry, solution, and gas-phase polymerizations. Certain applications, such as blow-moulding, require a careful balance of rheological properties, and there is a continuing effort to develop polyethylene with improved properties.[0003]A property that is especially important for...

AI Technical Summary

Benefits of technology

The polyethylene resin described in this patent has improved properties for making tight-head drums and fuel tanks. It has a high weight average molecular weight and a narrow molecular weight distribution, which gives it better resistance to impact and low swell during processing. The resin also has good clarity and excellent mechanical properties.

Problems solved by technology

When a resin exhibits too little swell, it can become difficult or impossible to properly fill out the extremities of the mold, such as the handle of a blow-molded bottle.

In particular, identifying the relative effects of average mass and polydispersity on swell proved difficult, as shown by Doelder et al Journal of Non-Newtonian Fluid Mechanics 152 (2008) 195-202.

However, the disadvantages of this process / produced polyethylene are that the Charpy impact resistance needs still to be improved for blow-moulding resins, in particular for applications such as tight-head drums.

While some properties can be improved by crosslinking with peroxides, there are issues with this approach.

The radicals produced can interact deleteriously with other additives.

Furthermore, it is difficult to predict the effect of crosslinking on rheological properties.

Peroxides add an extra component to the composition, and they require careful handling and storage, which adds to the overall cost.

U.S. Pat. No. 4,336,352 states that mixtures of high and low molecular weight polyethylene (or blends produced by a multi-step polymerization process) have such a low die swell that blow-molded bottles of consistent quality are difficult to obtain.

U.S. Pat. No. 5,422,400 states that earlier approaches with two-component mixtures have important limitations such as the need to use an ultrahigh molecular weight polyethylene having a minimum intrinsic viscosity.

Unfortunately, a three-component blend or a three-step polymerization is complicated and requires additional equipment.